Spotlight Finally on Privet

Tip necrosis symptoms on Chinese privet.

Privet (Ligustrum spp.) has a large native range, across Europe to eastern Asia and south to Queensland, Australia. There are approximately 40 species, but only four have been introduced to New Zealand: tree privet (L. lucidum), Chinese privet (L. sinense), common privet (L. vulgare), and Californian privet (L. ovalifolium).

Only tree privet and Chinese privet (referred to subsequently as privet) are considered to be seriously invasive weeds here currently. They are most common north of the Bay of Plenty, but also occur elsewhere throughout the North Island. Both species are only rarely found in the warmer parts of the South Island.

Privet can be highly invasive because it produces abundant dark bluish or purplish black berries that are eaten and dispersed by birds. In New Zealand bird dispersal of seeds is allowing privet to invade native plant communities where it suppresses regeneration of native species by reducing seedling survival and growth. Privet is also seriously unpopular with many people as its pollen can cause allergies including hay fever and asthma. Regional council phone lines ring red hot with complaints during the flowering season. This is slightly ironic given that privet was introduced to New Zealand as an ornamental and has been a popular hedge plant for many years in home gardens. The leaves of mature privet are also poisonous to stock.

Privet control is difficult because many of the habitats it invades preclude blanket spraying and make individual plants hard to find. Seedlings can be pulled or dug out, while older plants can be cut down, but the stumps must be treated with herbicide to prevent resprouting. Plants can also be poisoned by stem injection. Biocontrol potentially offers a much more cost-effective and sustainable method of control for privet. We looked into the feasibility of this back in 2000, but funds to begin a project did not become available until nearly a decade later. As usual our first step was to survey the plants in New Zealand to gain an appreciation of their current natural enemies here, which was completed recently. Between July 2009 and May 2012 we sampled three species of privet (L. lucidum, L. sinense and L. vulgare) from 39 New Zealand sites, ranging from Kaeo in the north of the North Island to Maruia and Granity in the South.

The result was typical of what we tend to find during such surveys. A wide range of native and introduced invertebrates are associated with privet in New Zealand but the damage caused is minimal (<2% of foliage currently consumed). Moth larvae, and in particular leafrollers (Tortricidae) and looper moths (Geometridae), appear to be causing the most obvious damage to privet foliage. One particularly interesting moth found (currently thought to be Trichophysetis cretacea) was reared from privet berries. Adults of this moth were first found in New Zealand in 1999 but its host plants were unknown until now. The larvae of this moth feed inside the berries and leave a neat round exit hole when they leave to pupate. The moth was not present at all sites, but where it was present it was sometimes causing a moderate amount of damage to berries. T. cretacea is native to China and Japan where it is known to be a pest of jasmine (Jasminium polyanthum), which is in the same family (Oleaceae) as privet. Further research is needed to determine whether they offer any potential for biocontrol purposes in New Zealand, especially given jasmine is both a very popular ornamental species and an up and coming weed in warmer parts of the country.

“Native puriri moth caterpillars (Aenetus virescens) are also quite common on privet, producing characteristic tunnels and feeding scars in the trunks,” explained Chris Winks, who did much of the actual survey work. Passionvine hopper (Scolypopa australis) and the green plant hopper (Siphanta acuta) were common at many sites but the damage caused by sap-feeders like these, either directly by removal of nutrients or indirectly by puncturing the plant and possibly allowing the entry of pathogens, is very difficult to quantify. The combined effect of generalist predators such as spiders, earwigs, ants, and praying mantids could inhibit the effectiveness of some potential invertebrate biocontrol agents for privet, and parasitoids identified during this survey could affect some potential lepidopteran biocontrol agents. These factors will need to be considered down the track when shortlists of potential biocontrol agents are being prepared.

Few primary fungal pathogens were found on privet, and the plant was healthier than many other weeds we have surveyed here. Symptoms observed included discrete leaf spots, tip and marginal scalds, but no obvious candidates for biocontrol purposes. Overall there appear to be no specialist privet natural enemies in New Zealand (apart from perhaps T. cretacea, whose host-range is not yet fully understood) and therefore there would appear to be good potential for improving privet control by introducing some. “We will look into this further and undertake screening of potential biocontrol agents once funds permit,” explained Stan Bellgard, who currently has overall responsibility for the project.

Some potential biocontrol agents are already known, as various privet species are considered to be weeds in the USA, Australia, Argentina, Mauritius and Réunion, so some work has already been undertaken. CABI Europe – UK has surveyed in Sri Lanka, India, Vietnam and China for potential biocontrol agents for L. robustum subsp. walkeri, which is problematic in Mauritius and Réunion. They identified three insect agents one of which, a moth (Epiplema albida), was found to be sufficiently host specific to be considered for release but has not yet been mobilised. Chinese privet has become one of the worst invasive plants in the south-eastern United States where it is considered a severe threat to ecosystems from Texas to Florida, and north as far as the New England states. Surveys for potential insect biocontrol agents for the USA were conducted in China during 2005 and 2006 with more than 100 species found feeding on the plant. The two thought to be the most promising, a leaf-mining flea beetle (Argopistes tsekooni) and a lacebug (Leptoypha hospita), have since been studied in more detail. James Hanula, of the US Forest Service, Georgia, reports that the beetle is probably not sufficiently specific to release in the USA and that they are currently completing testing of the lace bug and hope to request permission to release it shortly.

This project is funded by the National Biocontrol Collective. Thanks to Richard Toft (Entecol) for assistance with South Island surveys.